Optical devices

ABSTRACT

A solid-state, surface-emitting, optical device such as a light emitting diode (LED) or vertical cavity surface emitting laser (VCSEL) has a body of optical gain medium overlying a high reflectivity distributed BRAGG reflector (DBR) mirror which is carried on part of an underlayer. The gain layer is part of an epitaxial layered structure extending from the underlayer and over the mirror.

[0001] The present invention relates to improvements in or relating tosolid-state surface-emitting optical devices. In particular, theinvention relates to surface-emitting optical devices having structuresbased on the InAlGaN quaternary system, especially short-wavelength(less than 600 nm) Gallium Nitride (GaN) vertical-cavity surfaceemitting lasers (VCSELs) and GaN surface emitting diodes.

[0002] According to a first aspect of the present invention there isprovided a solid state, surface-emitting, optical device having a bodyof optical gain medium overlying a high reflectivity distributed Braggreflector (DBR) mirror which is carried by an underlayer,

[0003] wherein the DBR mirror is a multi-layer dielectric fabricationhaving alternate layers of dielectric material with a high refractiveindex ratio between the adjacent layers in the fabrication, and the bodyof optical gain medium is part of an epitaxial layered structureextending from the underlayer and over the fabrication.

[0004] By virtue of the DBR mirror being formed of dielectric material,the high refractive index ratio can be greater than 1.2; preferably, isgreater than 1.3; advantageously, is greater than 1.5, as a result ofwhich few periods (preferably, less than fifteen periods; advantageouslyless than ten periods) are required to produce a highly reflectivemirror (which, as is typical in laser devices, has a reflectivity of theorder of 97% or greater) which has the advantage that the fabricationprocess is simple.

[0005] Preferably, the fabrication is one of an array of columns havinga lateral dimension of less than approximately 50 μm and spaced apart(from centre to centre) by less than approximately 100 μm;advantageously, the columns have a lateral dimension of less thanapproximately 10 μm and are laterally spaced by less than approximately20 μm. Alternatively the fabrication may be one of an array of stripesor lines extending to a length of 100 μm or more, separated by a smallnumber of μm, typically about 10 μm, and having a width comparable indimension to the spacing.

[0006] It will be appreciated that the underlayer will usually be asubstrate having a buffer layer; preferably, the substrate is sapphire,alternatively, the substrate is SiC; preferably, the buffer layer isbased on any of the group three (periodic table) nitrides. If highquality substrates are available then the underlayer may consist of onlythe substrate.

[0007] The underlayer is typically a plate-like component with the DBRmirror fabrication carried by one surface and with the epitaxial layeredstructure extending from that surface. The surface may be planar withthe fabricated array of columns or stripes upstanding from the planarsurface. Alternatively, the surface may be patterned to form columnar orstriped depressions in which the fabricated mirror array is located. Ineach case the epitaxial layered structure extends from the surface andover the fabrication. In the limiting case the depressions extendthrough the thickness of the component and the DBR mirror fabrication iscarried by both the component and the epitaxial layered structure.

[0008] The epitaxial structure is formed by combinations from theInAlGaN quaternary system, for example, GaN or alloys thereof.Preferably, the epitaxial structure includes an Indium GalliumNitride-based (InGaN) multi-quantum well structure. Such epitaxialstructures are variously referred to as homo-epitaxial andhetero-epitaxial.

[0009] Preferably, one of the alternate layers in the multi-layerdielectric fabrication is silicon dioxide (SiO₂) and the other alternatelayer is titanium dioxide (TiO₂). The SiO₂/TiO₂ combination has a veryhigh refractive index ratio (approximately 1.6) and is particularlysuitable for operation near the 450 nm wavelength where absorption isvery low. Other suitable dielectric layers may be used, however, andthese include: MgF₂, CaF₂, Al₂O₃, ZnS, AlN. SiC, Si₃N₄ and ZrO₂; incombinations such as: SiO₂/SiC, SiO₂/Si₃N₄, CaF₂/ZnS, Al₃O₃/TiO₂,SiO₂/AlN, and SiO₂/ZrO₂. The SiO₂/ZrO₂ combination is particularlysuited to operation at about the 400 nm wavelength and has a refractiveindex radio of about 1.4.

[0010] Preferably, the body of optical gain medium is surmounted by aconductively-doped layer and overlies a conductively-doped layersurmounting the DBR mirror and electrodes are connected to theconductively-doped layers for electrical activation of the device,whereby the device is operable as a diode.

[0011] Preferably, a further mirror which is partially opticallytransmissive is disposed on the epitaxial structure in registration withthe DBR mirror so that the epitaxial structure functions as a solidstate optical cavity.

[0012] Where the optical device is a light-emitting diode, the furthermirror has a reflectivity in the range from approximately 50% to 90%, sothat lasing is not initiated. Where the optical device is a VCSEL, thefurther mirror has a reflectivity higher than approximately 98%, so thatlasing is initiated and, provided that the underlayer is transmissive,the lasing output may be taken either through the DBR mirror or thefurther mirror according to the respective reflectivities.

[0013] The further mirror may be made of any convenient materials, suchas semiconductors, metals and/or dielectrics.

[0014] According to a second aspect of the present invention there isprovided a method of fabricating a solid-state, surface-emitting,optical device incorporating an improved distributed Bragg reflector(DBR) mirror, the method comprising the steps of:

[0015] providing an underlayer;

[0016] growing a multi-layer coating on the underlayer, the coatingcomprising alternate layers of high refractive index dielectric and lowrefractive index dielectric;

[0017] selectively removing portions of the coating to provide an arrayof free-standing dielectric fabrications whereby portions of theunderlayer are revealed between adjacent fabrications;

[0018] epitaxially growing a semiconductor layered structureincorporating a body of optical gain medium on the revealed portions ofthe underlayer so that a lower part of the structure grows up andlaterally on top of the free-standing dielectric fabrications, and anupper part of the structure incorporates the body of optical gain mediumand overlies the fabrications so that one of the free-standingfabrications provides the DBR mirror.

[0019] By virtue of this aspect of the present invention, an efficientsurface-emitting optical device (such as a GaN VCSEL) incorporating aDBR mirror having few periods may be fabricated. The optical gain mediumoverlying the DBR mirror is substantially defect-free because the mirrorstops threading dislocations propagating from the underlayer. Becausethreading dislocations propagate vertically, the optical gain mediumabove the DBR is laterally offset from any threading dislocationspropagating from the underlying layer.

[0020] The method may further comprise the steps of

[0021] growing a further mirror on the body of optical gain medium;

[0022] providing a first electrode electrically connected to one side ofthe optical gain medium in registration with said one of thefree-standing fabrications; and

[0023] providing a second electrode electrically connected to theopposite side of the optical gain medium;

[0024] so that the optical gain medium functions as an optical cavitywhich may be electrically activated by the electrodes.

[0025] Conveniently the fabrication is in the form of an array ofindividual columns or of stripes (or lines) extending parallel to thecrystallographic direction <1, −1, 0.0> of the underlayer.

[0026] Preferably the array of fabrications is provided by patternetching. Alternatively the ‘lift off’ technique may be used whereby apattern of photo-resist material is deposited prior to the multi-layerdeposition coating and is subsequently chemically dissolved to removethe overlying multi-layer deposition and to leave the intervening areasof the multi-layer deposition which thereby form the column or stripedfabrications.

[0027] According to a third aspect of the present invention there inprovided a method of fabricating a solid-state surface-emitting opticaldevice incorporating an improved distributed Bragg reflector (DBR)mirror, the method comprising the steps of.:

[0028] providing an underlayer;

[0029] selectively patterning a surface of the underlayer to provide anarray of depressions in the surface;

[0030] providing an array of dielectric fabrications in the depressionswith portions of the underlayer revealed between adjacent fabrications,each fabrication comprising alternate layers of high refractive indexdielectric and low refractive index dielectric; epitaxially growing asemiconductor layered structure incorporating a body of optical gainmedium on the revealed portions of the underlayer so that a lower partof the structure grows up and laterally on top of the free-standingdielectric fabrications, and an upper part of the structure incorporatesthe body of optical gain medium and overlies the fabrications so thatone of the tree-standing fabrications provides the DBR mirror.

[0031] According to a fourth aspect of the present invention there isprovided a method of fabricating a solid-state surface-emitting opticaldevice incorporating an improved distributed Bragg-reflector (DBR)mirror, the method comprising the steps of:

[0032] providing an underlayer of gallium nitride;

[0033] patterning the underlayer with laser-drilled holes;

[0034] epitaxially growing a semi-conductor layered structureincorporating a body of optical gain medium on a surface of theunderlayer so that the lower part of the structure grows up andlaterally on the surface and overlies the holes therein; and

[0035] fabricating a multi-layer coating within the thickness of theholes so that the fabrications are carried by both the underlayer andthe epitaxial layered structure overlying the holes.

[0036] By selecting the optical gain medium the optical device mayoperate at wavelengths less than approximately 1 μm; in particular, byselecting an InGaN-based optical gain medium the optical device mayoperate at wavelengths less than 650 nm, with anticipated optimalperformance at approximately 400-450 nm.

[0037] These and other aspects of the present invention will be apparentfrom the following specific description, given by way of example, withreference to the accompanying drawings, in which:

[0038]FIGS. 1a to c illustrate three short-wavelength surface-emittingoptical devices in accordance with embodiments of the present invention;

[0039]FIGS. 2a to 2 h are schematic diagrams of the optical device ofFIG. 1b after various fabrication stages;

[0040]FIG. 2i is a schematic diagram of the device of FIG. 1b after thefabrication process is completed;

[0041]FIG. 3 is a graph of the calculated peak reflectivity versusnumber of periods for a DBR mirror used in the devices of FIGS. 1a to c;

[0042]FIG. 4 is a graph of the reflectivity versus wavelength for theDBR mirror used in the device of FIGS. 1a to c;

[0043]FIG. 5 is a schematic diagram illustrating a part of FIGS. 1a to 1c; and

[0044]FIG. 6 is a schematic diagram of the device of FIG. 1c.

[0045]FIG. 1a illustrates an electrically injected, GaN-based,solid-state surface-emitting optical device 10 a in accordance with oneembodiment of the present invention. The device 10 a is ashort-wavelength light-emitting diode 10 a. The diode 10 a has anunderlayer in the form of a substrate 12 with a buffer layer 13epitaxially grown thereon, a DBR mirror 14 disposed on part of thebuffer layer 13, and a layered structure 15 disposed on both the mirror14 and the buffer layer 13 so that the mirror is buried by the layeredstructure 15.

[0046] The layered structure 15 comprises a preparation layer (a firstconductive layer) 16, a body of optical gain medium 18 disposed on thepreparation layer, and a second conductive layer 20 disposed on the gainmedium 18.

[0047] The preparation layer 16 is disposed on and around the mirror 14so that the preparation layer 16 extends from the buffer layer 13, upthe sides of the mirror 14 and laterally on top of the mirror 14.

[0048] The diode 10 a also has a first electrode 22 electricallyconnected to one side of the optical gain medium 18 via the preparationlayer 16, and a second electrode 24 electrically connected to theopposite side of the optical gain medium 18 via the second conductivelayer 20.

[0049] In use, a forward bias is applied to the optical gain medium 18via the first and second electrodes 22,24. This potential causesgeneration of photons in the gain medium 18 and emission of thesephotons through the top surface 18 a of this medium 18 as shown byarrows 26. Photons emitted through the bottom surface 18 b of the medium18 are reflected by the mirror 14 so that they exit the diode 10 athrough the top surface 18 a.

[0050]FIG. 1b illustrates an electrically injected, GaN-based,solid-state microcavity surface-emitting optical device 10 b inaccordance with another embodiment of the present invention. In thisembodiment, the device 10 b is a short-wavelength VCSEL device; althougha similar structure could be used as a microcavity LED. The VCSEL 10 bis similar to diode 10 a, the difference being that the VCSVL 10 b has asecond mirror 28 (marginally less reflective than the first mirror)disposed on top of the second conductive layer 20 in registration withthe mirror 14. A microcavity LED would typically have a top mirror oflower reflectivity than a VCSEL, device would have.

[0051] In use, when a potential is applied to the gain medium (which isan optical cavity) 18 via the first and second electrodes 22,24, thispotential causer lasing within the cavity 18 and emission of coherentshort-wavelength radiation from the surface of the VCSEL 10 b via thesecond (top) mirror 28 as shown by arrow 30. Of course, if mirror 14were marginally less reflective than mirror 28 the primary emissionwould be through the substrate 12.

[0052]FIG. 1c illustrates a GaN-based, solid-state microcavitysurface-emitting optical device 10 c in accordance with anotherembodiment of the present invention. In this embodiment, the device 10 cis a short-wavelength optically-pumped VCSFL device 10 c. Theoptically-pumped VCSEL 10 c is similar to VCSEL 10 b; the differencebeing that VCSEL 10 c does not have any electrodes or a secondconductive layer (that is, the layered structure 15 comprises thepreparation layer 16, and the gain medium 18). VCSEL 10 c is pumped byoptical radiation incident on the surface of the VCSEL 10 c, as shown byarrow 31.

[0053]FIGS. 2a to 2 h are schematic diagrams of the structure of VCSEL10 b at various fabrication stages. The VCSEL 10 b emitsshort-wavelength light at some specified wavelength, typically in therange 400-450 nm.

[0054] Referring to FIG. 2a, the VCSEL 10 b is epitaxially grown aslayers on a sapphire substrate 12. A GaN buffer layer 13 approximately0.5 μm thick is grown on the sapphire substrate 12. A dielectricmulti-layer coating 32 comprising alternate layers of silica (SiO₃) 42and Titanium Dioxide (TiO₂) 44 is then grown on the GaN buffer layer 13.

[0055] The refractive index of silica at 450 nm is approximately 1.55and the refractive index of Titanium Dioxide at 450 nm is approximately2.81, giving a refractive index ratio of approximately 1.8. These valuesindicate that to obtain a quarter wavelength DBR mirror at, for example,450 nm the respective thicknesses of the silica layer 42 and the TiO₂layer 44 should be approximately 72.5 nm and 40 nm, which are therespective thicknesses grown in the multi-layer coating 32.

[0056]FIG. 3 shows; a graph of the calculated peak reflectivity versusnumber of periods of SiO₂/TiO₂ for the multi-layer coating 32 Comprisinga 72.5 nm thick SiO₂ layer 42 and a 40 nm TiO₂ layer 44. The peakreflectivity increases rapidly because of the very large refractiveindex ratio (1.8), so that 99% reflectivity is achieved for only fiveperiods of SiO₂/TiO₂. To ensure that high enough reflectivity (greaterthan approximately 99%) is achieved, six periods are used in multi-layercoating 32 (although for clarity only three periods are shown in FIGS.2a to 2 i).

[0057]FIG. 4 is a graph of the full reflectivity versus wavelength bandfor the six period multi-layer coating of FIG. 3. FIG. 4 shows that thereflectivity is very high across the spectral range from 425 nm to 475nm.

[0058] Referring to FIG. 2b and also to FIG. 2c (which is a plan view ofFIG. 2b), the six period multi-layer coating 32 is pattern-etched usingconventional photolithographic and etching techniques to create an arrayof free-standing columns 50, each column 50 having a lateral dimensionof approximately 5 μm and adjacent columns 50 being spaced approximately10 μm apart (between adjacent centres). Patterning the array of columnsreveals portions of the buffer layer 13 between adjacent columns 50. Anyone of these columns 50 may be selected for use as the mirror 14.

[0059] Referring to FIG. 2d, a preparation layer, in the form of a layerof n-doped GaN 16 in then grown on the areas of the buffer layer 13between the columns 50 so that the n-doped layer 16 grows up from theGaN buffer layer 13 until the top of the columns 50 is reached and thenthe layer 16 grows vertically and laterally on top of the columns 50 sothat the laterally-grown GaN coalesces to form a continuous n-doped GaNlayer 16.

[0060] The n-doped GaN layer 16 (preparation layer) is substantiallydefect-free as a result of this pattern-etching and growth technique; inparticular, the areas directly above the columns are substantially freefrom threading dislocations which propagate vertically from the bufferlayer 13. The n-doped layer 16 completely surrounds the columns 50,causing the columns 50 to be buried under the n-doped layer 16.

[0061] Referring to FIG. 2e, an optical cavity (microcavity) 18 is thengrown on the n-doped layer 16. This microcavity 18 has anInGaN/GaN/AlGaN active region, one example of which is shown in FIG. 5.The cavity as comprises: an a-doped In_(0.1)Ga₀₋₉N layer 52, an n-dopedAl_(0.14)Ga_(0.86)N/GaN modulation-doped strained-layer superlattice(MD-SLS) layer 54, an n-doped GaN layer 56, anIn_(0.02)Ga_(0.98)N/In_(0.15)Ga_(0.85)N multi-quantum well layer 58, ap-doped Al_(0.2)Ga_(0.8)N layer 60, a p-doped GaN layer 62, and ap-doped Al_(0.14)Ga_(0.86)N/GaN MD-SLS layer 64.

[0062] Referring to FIG. 2f, a p-doped GaN layer 20 is then grown on thetop surface 18 a of the microcavity 18 subsequently, as shown in FIG.2g, an area laterally spaced from one of the columns 50A (which is thecolumn selected to function as the DBR mirror 14) is then etched away sothat a portion of the n-doped layer 16 is revealed. An electrode 22composed of Titanium and Aluminium is deposited onto the revealedportion of the n-doped layer 16, so that the electrode 22 is laterallyspaced from the column 50A (which is mirror 14). This electrode 22 isused as the n-electrode.

[0063] Referring to FIG. 2h, a layer of silica 72 is then grown on thep-doped layer 20. This silica layer 72 is then patterned and etched sothat a second electrode 24, made of Gold and Nickels, may be depositedonto conductive layer 20 in the etched areas of silica. This electrode24 is used as the p-electrode. The p-electrode defines an aperture whichis in registration with the mirror 14 and cavity 18.

[0064] The p-electrode 24 is electrically connected to the top surface18 a of the microcavity 18 via the p-doped layer 20, and the n-electrode22 is electrically connected to the bottom surface 18 b of microcavity18 by connection to the n-doped layer 16.

[0065] A second mirror 28 is then deposited on the top of p-doped layer20 at an area vertically above the mirror 14 and the microcavity 18. Thesecond mirror 28 is a dielectric mirror coating which is similar tocoating 32 but only has five periods so that the reflectivity of mirror28 is marginally less than that of coating 32. A second differencebetween coating 32 and mirror 28 is that mirror 28 is notpattern-etched.

[0066]FIG. 2i shows the complete VCSEL 10 b, however for clarity onlycolumn 50A is shown. In use, carriers are electrically injected into themicrocavity 16 by applying a voltage to the n- and p- electrodes 22,24.The mirrors 14,28 provide very high reflectivity so that, in use, highintensity coherent light of approximately 450 nm wavelength is emittedfrom the top of the VCSEL 10 b (through the mirror 28) as shown by arrow30.

[0067]FIG. 6 is a schematic diagram of the structure of the opticallypumped VCSEL 10 c of FIG. 1c. The VCSEL 10 c also emits short-wavelengthlight at a specified wavelength in the range 400-450 nm.

[0068] VCSEL 10 c is similar to VCSEL 10 b; however, there are noelectrodes in VCSEL 10 c. VCSEL 10 c has a sapphire substrate 12, a GaNbuffer layer 13, a DBR mirror 14 (formed from a pattern-etchedsix-period, SiO₂/TiO₂ dielectric coating), an n-doped GaN layer 16, anda microcavity 18, all identical to those of FIG. 2i. However, the secondmirror 28 is disposed directly on the top of the microcavity 18 (thatis, there is no intermediate conductive layer). In this embodiment,carriers are generated in the microcavity 18 by illuminating the top ofthe VCSEL 18 c by a pump beam (as shown by arrow 31) of suitablewavelength and intensity.

[0069] Where a GaN light emitting diode is to be fabricated, thestructure of FIG. 2i may be fabricated without the top mirror 28, asthis is not required for LED operation. Alternatively, a top mirror maybe used which in not highly reflective (only partially reflective), sothat some radiation would be reflected back to cavity 18 but notsufficient radiation to cause lasing in the cavity. The GaN LED wouldemit Short-wavelength light, for example, centred on approximately 450nm. The GaN LED may use more than one (for example, an array of onehundred) of the columns 50 to provide a DBR mirror function.

[0070] Various modifications may be made to the above describedembodiments For example, the p- and n-electrodes may be fabricated usingdifferent materials than those described. In other embodiments thecolumns may be stripes (lines), rather than the hexagons shown in FIG.2c, in which care only a portion of the stripe length is used to formthe mirror and conveniently the electrode 22 is deposited over adifferent portion of the same stripe (so as to be located on asubstantially defect-free region of layer 16). Substrates other thansapphire may be used, for example silicon carbide may be used. It willbe appreciated that an array of surface-emitting devices may befabricated on a single substrate. Furthermore, in the manufacturingmethod the underlayer may first be patterned to provide an array ofdepressions in the surface and thereafter the dielectric fabricationsmay be deposited in the depressions with portions of the underlayerrevealed between adjacent fabrications, the epitaxial structure thenbeing grown on the revealed portions. Alternatively the patterning ofdepressions may take the form of laser drilled holes (circular orelongate) with the epitaxial structure then being grown on the aperturedunderlayer which preferably is high quality Gallium Nitride (GaN) andwith the Bragg mirror fabrications subsequently being formed in theholes so that the fabrications are carried both by the underlayer and bythe epitaxial layered structure overlying the holes.

1. A solid state, surface-emitting, optical device having a body ofoptical gain medium overlying a high reflectivity distributed Braggreflector (DBR) mirror which is carried by an underlayer, wherein themirror is a multi-layer dielectric fabrication having alternate layersof dielectric material with a high refractive index ratio between theadjacent layers in the fabrication, and the body of optical gain mediumis part of an epitaxial layered structure extending from the underlayerand over the fabrication.
 2. A device as claimed in claim 1, wherein thehigh refractive index ratio is greater than 1.3.
 3. A device as claimedin either preceding claim, wherein the underlayer comprises a substratehaving a buffer layer which is a nitride of a group three element in theperiodic table.
 4. A device as claimed in any preceding claim, whereinthe epitaxial structure is formed by combinations from the InAlGaNquaternary system.
 5. A device as claimed in claim 4, wherein theepitaxial structure includes an Indium Gallium Nitride-based (InGaN)multi-quantum well structure.
 6. A device as claimed in any precedingclaim, wherein a further mirror which is partially opticallytransmissive is disposed on the epitaxial structure in registration withthe DBR mirror so that the epitaxial structure functions as a solidstare optical cavity.
 7. A method of fabricating a solid-state,surface-emitting, optical device incorporating an improved distributedBragg reflector (DBR) mirror, the method comprising the steps of:providing an underlayer; growing a multi-layer coating on theunderlayer, the coating comprising alternate layers of high refractiveindex dielectric and low refractive index dielectric; selectivelyremoving portions of the coating to provide an array of free-standingdielectric fabrications whereby portions of the underlayer are revealedbetween adjacent fabrications; epitaxially growing a semiconductorlayered structure incorporating a body of optical gain medium on therevealed portions of the underlayer so that a lower part of thestructure grows up and laterally on top of the free-standing dielectricfabrications, and an upper part of the structure incorporates the bodyof optical gain medium and overlies the fabrications so that one of thefree-standing fabrications provides the DBR mirror.
 8. A method asclaimed in claim 7, comprising the steps of growing a further mirror onthe body of optical gain medium; providing a first electrodeelectrically connected to one side of the optical gain medium inregistration with said one of the free-standing fabrications; andproviding a second electrode electrically connected to the opposite sideof the optical gain medium; so that the optical gain medium functions asan optical cavity which may be electrically activated by the electrodes.9. A method of fabricating a solid-state surface-emitting optical deviceincorporating an improved distributed Bragg reflector (DBR) mirror, themethod comprising the steps of; providing an underlayer; selectivelypatterning a surface of the underlayer to provide an array ofdepressions in the surface; providing an array of dielectricfabrications in the depressions with portions of the underlayer revealedbetween adjacent fabrications, each fabrication comprising alternatelayers of high refractive index dielectric and low refractive indexdielectric; epitaxially growing a semiconductor layered structureincorporating a body of optical gain medium on the revealed portions ofthe underlayer so that a lower part of the structure grows up andlaterally on top of the free-standing dielectric fabrications, and anupper part of the structure incorporates the body of optical gain mediumand overlies the fabrications so that one of the free-standingfabrications provides the DBR mirror.
 10. A method of fabricating asolid-state surface-emitting optical device incorporating an improveddistributed Bragg-reflector (DBR) mirror, the method comprising thesteps of: providing an underlayer of gallium nitride; patterning theunderlayer with laser-drilled holes; epitaxially growing asemi-conductor layered structure incorporating a body of optical gainmedium on a surface of the underlayer so that the lower part of thestructure grows up and laterally on the surface and overlies the holestherein; and fabricating a multi-layer coating within the thickness ofthe holes so that the fabrications are carried by both the underlayerand the epitaxial layered structure overlying the holes.